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MRI when X-rays, ultrasounds, and other more “traditional” diagnostic tools have failed to find out what and where the problem is when lameness persists. In these cases, identifying the exact location and cause of the injury can be a time-consuming task that impacts the horse’s training schedule. Enter MRI, a powerful tool that uses a magnet field to create detailed images (called “slices”) of every millimeter of the limb in virtually every possible direction to find those hard-to-diagnose injuries. Although originally used strictly as a research tool, “MRI is being used with increasing frequency and is considered a valuable tool for diagnosing certain types of lameness, including those commonly seen in Thoroughbreds,” relays Sue Dyson, MA, VetMB, PhD, DEO, FRCVS, head of clinical orthopedics at the Animal Health Trust in Newmarket, England. Michael Ross, DVM, Dipl. ACVS, a professor of surgery at the University of Pennsylvania's New Bolton Center, agrees with Dyson and adds, “The most important region imaged is the fetlock (ankle), and MRI is the tool of choice for diagnosing maladapative bone remodeling.” Ross is referring to a condition also called “bone bruising,” which was previously covered in Issue 19 of North American Trainer. Briefly, bone bruising, also called “the starting back blues,” is a chronic injury of the cannon bone at the bottom of the fetlock joint caused by repetitive stress. In addition to bone bruising, this article looks at other important causes of lameness that can accurately and quickly be diagnosed in Thoroughbred racehorses. Some important “downsides” of MRI are also discussed.
Merits of MRI: what can it do? Bone bruising isn’t the only “mystery” lameness that MRI can pinpoint when X-rays are not conclusive. Both Ross and Dyson agree that the MRI is also extremely valuable in identifying other conditions of the fetlock joint, such as incomplete fractures of various bones. “Studies have shown that MRI of the fetlock joints of Thoroughbred racehorses can diagnose a variety of conditions that radiographs fail to diagnose,” relays Dyson. For example, in a recent study by Sarah E. Powell MA, VetMB, AECVDI, MRCVS, from Rossdales Equine Diagnostic Centre, Newmarket, United Kingdom, MRI was successfully used to diagnose injuries in 97.8% of the 131 horses (168 joints) scanned while sedated between October 2006 and August 2010. The most common injuries were: l Osteochondral disease within the joint (observed in 54.9% of horses); l Incomplete condylar fractures (19.8%); l Incomplete sagittal fracture of P1 and the pastern (14.5%); and l Soft tissue injuries (12.2%). One powerful observation made in this study was that in 35.8% of the horses, the MRI findings were consistent with the presence of a fracture that was not evident on X-rays. In addition, MRI
A sedated horse standing in the MRI scanner at Rossdale’s Diagnostic Centre, Newmarket. The right fore fetlock is positioned for a scan between the poles of the blue coloured magnet
failed to identify any significant bone or soft tissue injury in a mere 5.4% of these cases. “This study confirms that some incomplete fractures can be diagnosed via MRI that would not necessarily show up on traditional X-rays, and that performing an MRI scan in lame horses could potentially prevent catastrophic injury,” says Dyson. Suspensory branch desmitis is another
condition of the fetlock that is amenable to an MRI diagnosis. “The suspensory ligament runs from the back of the knee, down the canon bone to the sesamoid bones in the ankle (fetlock). In most instances suspensory desmitis is diagnosed based on palpation and the application of ultrasonographic imaging. However, early suspensory tears and those that are on the
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